1. Field of the Invention
This invention relates to a method for the production of a finely porous article having silica particles inserted between layers of smectite type mineral.
2. Prior Art Statement
Heretofore, as a means for producing a porous material, a method using a smectite type mineral and a metal salt has been attempted.
Examples of the smectite type mineral include montmorillonite, bentonite, chlorite, hectorite, beidellite, and synthetic mica.
They have one common attribute, which will be described below with reference to montmorillonite.
The monotmorillonite has the crystals thereof superposed in a three-sublayer structure of silicic acid tetrahedron-alumina octahedron-silicic acid tetrahedron to form each composite layer.
The surface of each composite layer is negatively charged with part of the alumina of the aforementioned alumina octahedron crystals substituted with magnesium, of small electric charge. The negative charge on the surface of the composite layer is neutralized because an alkali metal ion (Na.sup.+) corresponding to the negative charge intervenes between every composite layer.
The montmorillonite, therefore, has a large cation-exchange capacity. The interstices between the respective layers absorb a large volume of water through the reaction of Na.sup.+ hydration and swell to separate the component layers to a notable extent. This attribute is inherent also in the other smectite type minerals.
By virtue of this attribute, the smectite type minerals have found utility as raw materials for the production of catalysts, adsorbents, and even porous substances useful as separating agents.
For example, Japanese Patent Disclosures SHO 54(1979)-5884 and SHO 54(1979)-16386 disclose inventions relating to a method for manufacture of a porous clay smectite type mineral containing between the layers of the smectite type mineral a cationic hydroxy metal complex, aluminum chlorohydroxide complex, silicates, phosphates, and zirconia by mixing the smectite type mineral, a cationic inorganic substance, and water, causing the cationic inorganic substance to exchange ions with the exchanging cation between the crystal layers, and hydrolyzing the resultant composite. In this porous clay, the intervals between the crystal layers are not more than 10 .ANG..
The inventor has been granted U.S. Pat. No. 4,629,713 relating to a method for the production of a porous article containing pores of a larger diameter than those in the porous products of such conventional methods as described above.
This invention concerns a finely porous clay comprising crystal layers of a smectite type mineral separated by intervals of not less than 20 .ANG. and an inorganic substance and optionally a water-soluble polymer embraced between said crystal layers, and a method for the production of said finely porous clay comprising mixing a smectite type mineral, a water-soluble polymer, one member selected from the group consisting of silica colloidal and water glass, at least one cation, and water and subsequently removing the water-soluble polymer from the resultant mixture. In this invention, water-soluble polymers usable are limited to starch, konjak, agar, grated yam, gum arabic, glue, gelatin, polyethylene oxide, polyvinyl alcohol, polyacrylic acid derivatives, polyvinyl sulfonic acid derivatives, and carboxycellulose derivatives.
In the method of production described above, the silica colloidal intervenes between the crystal layers of smectite mineral and the water-soluble polymer plays the part of increasing the interfacial intervals between the layers.
However, the invention of the aforementioned U.S. Patent has the following disadvantage because the water-soluble polymer is limited as described above. The surface of the smectite mineral and that of the inorganic substance are equally charged negatively and, therefore, repel each other, while the positive charge at the edge of the smectite mineral and the negative charge of the inorganic substance attract each other. Thus, the inorganic substances are present not merely in the interfacial intervals but also around the particles formed by the layer stacks. Thus, the produced porous clay possesses a complicated structure. Because of its structural complexity, the porous clay inevitably contains micropores of diameters distributed over a wide range. It is therefore not suited for use as a separating material for separation of molecules of a specific diameter or as a catalyst for the production of specific molecules.
The porous article produced by this method, on being burnt, produces carbon because the polymer is present between the layers. The presence of this carbon is detrimental when the porous article is used as a catalyst, for example. The present inventors have perfected a method which is free from the disadvantage of the invention of the aforementioned U.S. patent and has filed for a U.S. patent on this improvement (U.S. Application Ser. No. 06/940,087). This improvement consists in using, in place of the water-soluble polymer essential to the aforementioned U.S. patent, a mixture of (1) molecular quaternary ammonium salt having a quaternary amine linked to a water-soluble macromolecular compound and possessing basicity, (2) a polymer of a polyoxyethylene alkylvinyl monomer and a vinyl monomer possessing basicity, or (3) a polymer of a halogenide of a quaternary amine, i.e. a basic macromolecular compound, and at least one water-soluble macromolecular substance selected from the group consisting of cellulose, polyvinyl alcohol, polyethylene oxide, and polyacrylamide.
The substances described above, however, are invariably expensive and the methods used therefor are complicated in terms of procedure. Thus, they entail problems in practical application to volume production of porous articles.